If you are allowed to "use your knowledge of the derivative", you could say that by definition the derivative of a function (here your function is x^-3.25) is the limit as h approaches 0 of f(x + h) - f(x). Therefore, this limit equals the derivative of x^-3.25 which is (and you know how to do the derivative, right?)

find the derivative of f(x)...forget about the h. I think your prof just want's you to see how easy it is to do limits in that form now that you can do derivatives. Or maybe he / she wants to reinforce how derivatives relate to limits.